Secular trends in the fat and fat-free components of body mass index in children aged 8-18 years born 1958-1995.

Background: It is unknown whether the secular trend in childhood BMI reflects increases in fat-free mass as well as fat mass. Methods: This study decomposed BMI trends in 488 participants in the Fels Longitudinal Study born between 1958-1995 and aged 8-17.99 years into their fat and fat-free components. Generalized estimating equations estimated birth year cohort (1958-1970, 1971-1983, 1984-1995) effects on 2208 observations of BMI, fat mass index (FMI = fat mass (kg)/height (m)(2)) and fat-free mass index (FFMI = fat-free mass (kg)/height (m)(2)). Results: BMI in boys increased across cohorts, with those born between 1984-1995 being 2 kg/m(2) larger than those born between 1958-1970 (p = 0.001) and increases in FMI were highly significant (p-values, 0.001). FFMI did not differ by cohort. In girls, there was a significant advantage in BMI (1.2 kg/m(2)) and FFMI (0.8 kg/m(2)) of the 1984-1995 cohort compared to the 1971-1983 cohort (p-values, 0.05). Conclusions: Because the long term trend in childhood BMI in boys appears to be driven by an increase in total body adiposity, evidence is provided to support current knowledge on the predicted deleterious long-term consequences of the childhood obesity epidemic in boys. Research is needed to confirm whether recent changes in BMI in girls are due to increases in fat-free mass resulting from changes in behaviour and lifestyle not yet manifest in boys.